1. Field of the Invention
The present invention relates to a transmitting apparatus, a communication system, a transmitting method and a program.
2. Description of the Related Art
Communication technology called orthogonal frequency division multiplexing (OFDM) is known in the wireless communication field or the like. The technology is a digital modulation technique that arranges a plurality of subcarriers at regular frequency intervals and transmits information through each subcarrier. According to this technology, a plurality of carriers can be arranged densely without interfering with each other so as to realize broadband transmission with effective use of a narrow frequency range. It is thereby possible to increase the frequency use efficiency and it is thus suitable for high-speed data transmission.
For example, the Institute of Electrical and Electronic Engineers (IEEE) 802.11a, which is one of wireless local area network (LAN) standards, wirelessly transmits data by the OFDM technology. Further, ultra wideband (UWB) technology, which is being promoted for the practical use, transmits a multicarrier signal using a high frequency such as the GHz band with a relatively wide bandwidth.
In the OFDM communication technology that uses a carrier in a relatively wide band, interference with a carrier used in another communication network (which is referred to hereinafter as an interfered system) possibly occurs. This occurs, for example, in the case where a cellular phone in another communication network performs communication using a prescribed frequency that is included in the frequency band of a multicarrier signal. Because the occurrence of such interference causes degradation of a communication quality and even leads to a failure in communication, it is necessary to take appropriate measures to prevent the occurrence of interference at the OFDM transmitting end. As one of the measures, technology called detect and avoid (DAA) has been studied recently. The DAA is a technique that, when transmitting a multicarrier signal, stops transmission of a particular frequency band (e.g. a subcarrier unit) in a relatively wide frequency band for wireless transmission.
Generally, when the existence of an interfered system is not detected, transmission is performed in the whole range of the frequency band for wireless transmission. However, if transmission is performed by the OFDM technology in the whole range of the frequency band for wireless transmission on the assumption that no interfered system exists, it may be impossible to detect the existence of an interfered system, when it exists, due to noise generated by the transmission. Further, if transmission is performed always using the whole range of the frequency band, it is difficult to perform communication by avoiding the frequency band of an interfered system in the case where there is an interfered system whose existence is not detected or an interfered system newly becomes active, for example.
To avoid this, a technique that detects the existence of an interfered system by setting a period (silent period (SP)) during which a transmission power is reduced for a given length of time at regular time intervals in subcarrier transmission in a frequency band allocated to an interfered system has been introduced. This technique enables reliable detection of the existence of an interfered system during the silent period.
FIGS. 10 and 11 are schematic views to explain the silent period. FIG. 10 is a schematic view showing a subcarrier before performing time domain transform by an inverse fast Fourier transformer (IFFT) of a transmitting apparatus in the frequency domain. As shown in FIG. 10, a frequency band (SB) in which an interfered system possibly exists is specified in advance. The frequency band of an interfered system varies by country and region, and in the WiMAX standard in Europe, for example, it is mandatory to reduce a transmission power in a prescribed frequency band when the band is used by the WiMAX (interfered system).
If the existence of an interfered system is not detected, a signal is transmitted in the whole frequency range as shown in FIG. 10(a). On the other hand, a transmission power is reduced in the frequency band SB in which an interfered system possibly exists as shown in FIG. 10(b).
The reduction of a transmission power is performed at regular time intervals. FIG. 11 is a schematic view showing the silent period in the time domain after performing time domain transform by the IFFT in the transmitting apparatus. In FIG. 11, the reduction of a transmission power is performed during the silent period SP (the portion indicated by dots in FIG. 11). Each regular interval including the silent period SP is referred to as SPC, and transmission is performed without reducing a transmission power as shown in FIG. 10(a) during a non-silent period (NSP) in the SPC. On the other hand, transmission is performed by reducing a transmission power in the frequency band SB in which an interfered system possibly exists as shown in FIG. 10(b) during the silent period SP in the SPC. The silent period time SP, the interval SPC, and the frequency band SB in which transmission is to be avoided may be regulated by the legal system of the country in which an interfered system exists.
Setting the silent period prevents noise in the OFDM transmission from affecting the detection of an interfered system when the interfered system exists. It is thereby possible to reliably detect the interfered system during the silent period. After detecting the interfered system, transmission is stopped only in the particular frequency band used by the interfered system by the DAA technology, thereby suppressing the effect on the interfered system.    [Patent document 1] Japanese Unexamined Patent Application Publication No. 2007-166068    [Patent document 2] Japanese Unexamined Patent Application Publication No. 2007-258904    [Patent document 3] Japanese Unexamined Patent Application Publication No. 2007-243235    [Patent document 4] Japanese Unexamined Patent Application Publication No. 2007-243236